Mark some data during drag-and-drop

Mark some data during drag-and-drop

It doesn't make sense to prevent the return frame or movement
frame from being deleted, but we should at least protect them
from garbage collection.

* src/alloc.c (garbage_collect): Call mark_xterm.
* src/xterm.c (x_dnd_begin_drag_and_drop)
(x_dnd_cleanup_drag_and_drop): Clear movement and return frames
upon DND completion.
(mark_xterm): Mark those frames.
* src/xterm.h: Update prototypes.